Stent installation method using balloon catheter having stepped compliance curve

ABSTRACT

A method for installing a stent in a vessel utilizes a single balloon catheter for both low pressure predilation at a relatively small diameter to open the lesion sufficiently to allow insertion and deployment of the stent across the lesion and for subsequent high pressure embedding of the stent in the vessel wall. The same balloon catheter may also be employed to insert and deploy the stent. The balloons utilized in the method have a stepped compliance curve which allows for predilation at a low pressure and predetermined diameter and for high pressure embedding at a substantially larger diameter. The balloons may be provided with a configuration in which only a portion of the balloon has a stepped compliance curve while a further portion has a generally linear compliance profile. With such balloons high pressure treatment of the vessel wall areas not reinforced by the stent can be avoided despite the occurence of longitudinal shrinkage of the stent during expansion thereof.

This is a division of application Ser. No. 09/396,841, filed Sep. 15,1999 now U.S. Pat. No. 6,290,485, which is a continuation of applicationSer. No. 08/931,190, filed Sep. 16, 1997, now U.S. Pat. No. 5,980,532,which is a division of application Ser. No. 08/397,615 Mar. 2, 1995, nowU.S. Pat. No. 5,749,851.

BACKGROUND OF THE INVENTION

The present invention relates generally to a method of installing astent utilizing a balloon catheter to perform an initial angioplasty andto seat the stent after it has been located in the vessel. The inventionalso relates to novel balloon structures which have particular use inthe method of the invention.

Angioplasty, an accepted and well known medical practice involvesinserting a balloon catheter into the blood vessel of a patient,maneuvering and steering the catheter through the patient's vessels tothe site of the lesion with the balloon in an uninflated form. Theuninflated balloon portion of the catheter is located within the bloodvessel such that it crosses the lesion or reduced area. Pressurizedinflation fluid is metered to the inflatable balloon through a lumenformed in the catheter to thus dilate the restricted area. The inflationfluid is generally a liquid and is applied at relatively high pressures,usually in the area of six to twenty atmospheres. As the balloon isinflated it expands and forces open the previously closed area of theblood vessel. Balloons used in angioplasty procedures such as this aregenerally fabricated by molding and have predetermined design dimensionssuch as length, wall thickness and nominal diameter. Balloon cathetersare also used in other systems of the body for example the prostate andthe urethra. Balloon catheters come in a large range of sizes and mustbe suitably dimensioned for their intended use.

Recently the use of a catheter delivered stent to prevent an openedlesion from reclosing or to reinforce a weakened vessel segment, such asan aneurism, has become a common procedure. A typical procedure forstent installation involves performing an initial angioplasty to openthe vessel to a predetermined diameter sufficent to permit passage of astent delivery catheter across the lesion, removal of the angioplastyballoon catheter, insertion of a delivery catheter carrying the stentand a stent deploying mechanism, deploying the stent across the openedlesion so as to seperate the stent from the catheter and bring it intocontact with the vessel wall, usually with dilation to a larger diameterusing a balloon larger than the balloon of the predilation catheter, andthen removing the delivery catheter (after deflating the balloon ifused). In many cases it has become the practice to then “retouch” thedilation by deploying a third catheter carrying a balloon capable ofdilating at a substantially higher pressure to drive the stent into thevessel wall, thereby to assure that there is no risk of the stent latershifting its position and to reduce occurance of restenosis or thrombusformation. This third “retouch” dilation is often considered necessarywhen the balloon used to seat the stent is made of a compliant materialbecause such balloons generally cannot be safely pressurized above 9-12atm., and higher pressures are generally considered necessary to assurefull uniform lesion dilation and seating of the stent.

A wide variety of stent configurations and deployment methods are known.For instance, stent configurations include various forms of bent wiredevices, self-expanding stents; stents which unroll from a wrappedconfiguration on the catheter; and stents which are made of a deformablematerial so that the device may be deformed on deployment from a smalldiameter to a larger diameter configuration. References disclosing stentdevices and deployment catheters include:

US 4733665 Palmaz US 4681110 Wiktor US 4776337 Palmaz US 4800882Gianturco US 5195984 Schatz US 4830003 Wolff et al US 5234457 AndersenUS 4856516 Hillstead US 5116360 Pinchuck et al US 4922905 Strecker US5116318 Hillstead US 4886062 Wiktor US 4649922 Wiktor US 4907336Gianturco US 4655771 Wallsten US 4913141 Hillstead US 5089006 Stiles US5092877 Pinchuk US 5007926 Derbyshire US 5123917 Lee US 4705517 DiPisa,Jr. US 5116309 Coll US 4740207 Kreamer US 5122154 Rhodes US 4877030 Becket al US 5133732 Wiktor US 5108417 Sawyer US 5135536 Hillstead US4923464 DiPisa, Jr. US 5282824 Gianturco US 5078726 Kreamer US 5292331Boneau US 5171262 MacGregor US 5035706 Gianturco et al US 5059211 Stacket al US 5041126 Gianturco US 5104399 Lazarus US 5061275 Wallsten et alUS 5104404 Wolff US 5064435 Porter US 5019090 Pinchuk US 5092841 SpearsUS 4954126 Wallsten US 5108416 Ryan et al US 4994071 MacGregor US4990151 Wallsten US 4580568 Gianturco US 4990155 Wilkoff US 4969890Sugita et al US 5147385 Beck et al US 4795458 Regan US 5163952 Froix US4760849 Kropf US 5192297 Hull

In U.S. Pat. No. 5,348,538, incorporated herein by reference, there isdescribed a single layer balloon which follows a stepped compliancecurve. The stepped compliance curves of these balloons has a lowerpressure segment following a first generally linear profile, atransition region, typically in the 8-14 atm range, during which theballoon rapidly expands yielding inelastically, and a higher pressureregion in which the balloon expands along a generally linear, lowcompliance curve. The stepped compliance curve allows a physician todilate different sized lesions without using multiple balloon catheters.

Stepped compliance curve catheter balloon devices using two differentcoextensively mounted balloon portions of different initial inflateddiameter, are also described in co-pending U.S. application Ser. No.08/243,473, filed May 16, 1994 as a continuation of now abandoned U.S.application Ser. No. 07/927,062, filed Aug. 8, 1992, and in U.S. Pat.No. 5,358,487 to Miller. These dual layer balloons are designed with theouter balloon portion larger than the inner portion so that thecompliance curve follows the inner balloon portion until it reachesburst diameter and then, after the inner balloon bursts, the outerballoon becomes inflated and can be expanded to a larger diameter thanthe burst diameter of the inner balloon.

A polyethylene ionomer balloon with a stepped compliance curve isdisclosed in EP 540 858. The reference suggests that the balloon can beused on stent delivery catheters. The disclosed balloon material of thisreference, however, yields a compliant balloon and therefore a stentdelivered with such a balloon would typically require “retouch.”

SUMMARY OF THE INVENTION

The invention in one aspect is directed to a method for method forinstalling a stent in a vessel utilizes a single balloon catheter forboth low pressure predilation at a relatively small diameter to open thelesion sufficiently to allow insertion and deployment of the stentacross the lesion and for subsequent high pressure embedding of thestent in the vessel wall. The same balloon catheter may also be employedto insert and deploy the stent. Thus at least one catheter may beeliminated from what has heretofore been a two or three catheterinstallation process. The balloons utilized in the method have a steppedcompliance curve which allows for predilation at a low pressure andpredetermined diameter and for high pressure embedding at asubstantially larger diameter.

In a further aspect of the invention novel balloon structures havinghigh wall strengths, high burst pressures and low compliance areprovided in which a first portion of the balloon body has a generallylinear compliance curve and a second portion of the balloon body has astepped compliance curve. Both portions of the balloon are configured tohave essentially the same diameter at low pressure so that the entireballoon may be used to predilate a lesion. However at higher pressurethe configuration of the balloon changes due to rapid expansion of thesecond balloon portion. At still higher pressures the compliance curveof the second portion levels off to a low compliance profile so thatthis portion of the balloon can be used for high pressure embedment ofthe stent without substantially increasing the stent size. With suchballoons, exposure of the vessel wall areas which are not reinforced bythe stent to high pressure can be avoided, despite the typically shorterlength of conventional stents than the typical length of predilationballoons.

The novel balloons of the invention are made by molding a balloon into aconfiguration in which the second portion has a larger diameter than thefirst portion and then shrinking the second portion to the diameter ofthe first portion. The method of making such balloons comprises yetanother aspect of the invention.

These and other aspects and advantages of the present invention will nodoubt become apparent to those skilled in the art after having read thefollowing detailed description of the invention as illustrated by thevarious drawing figures.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a longitudinal sectional view of a vessel showing anangioplasty catheter, not in section and having a stepped compliancecurve balloon on the distal end thereof, inserted in the vessel andpredilating a lesion in the vessel.

FIG. 2 is a view of a vessel as in FIG. 1 after installation of a stentbut before a “retouch” procedure.

FIG. 3 is a view as in FIG. 1 in which after predilation and with thesame catheter, now carrying a stent mounted over the balloon, reinsertedto deliver the stent to the lesion.

FIG. 4 is a view as in FIG. 3 with the balloon expanded to install thestent and further dilate the lesion.

FIG. 5 is a view as in FIG. 3 after completion of the procedure of FIG.3.

FIG. 6 is a side view the distal end of a catheter having an alternateballoon of the invention, shown in hyper-extended form.

FIG. 7 is a schematic illustration depicting the process stages forpreparing a balloon as in FIG. 6.

FIG. 8 is a view of a catheter as in FIG. 6 except that a secondalternate balloon of the invention is depicted.

FIG. 9 is a schematic illustration depicting the process stages forpreparing a balloon as in FIG. 8.

FIG. 10 is a graph showing the compliance curves of several balloons ofthe type shown in FIGS. 1, 3 and 4 compared to a conventional 3.5 mmangioplasty balloon of the same material.

FIG. 11 is a graph of the compliance curves of a balloon of the typeshown in FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The catheters employed in the practice of the present invention are mostconveniently constructed as over-the-wire balloon catheters ofconventional form for use in angioplasty, except that the balloon has astepped compliance curve. However it should be understood that thepresent invention can be applied, in addition to over-the-wirecatheters, to fixed-wire catheters, to shortened guide wire lumens orsingle operator exchange catheters, and to non over-the-wire ballooncatheters. Furthermore this invention can be used with balloon cathetersintended for use in any and all vascular systems or cavities of thebody.

Referring to FIGS. 1-5, the process of the invention is illustrated bythese Figures. In FIG. 1, a catheter 10 carrying a balloon 12 on thedistal end thereof has been inserted over guide wire 13 into a vessel 14and fed to a lesion 16 where it is used to predilate the lesion to apredetermined diameter, typically about 2.5 mm. In the process of theinvention, balloon 12 is made of a high strength polymer, such as PETand has a stepped compliance curve, the predilation diameter is belowthe transition region on that curve and the desired final dilateddiameter, typically 2.75-4.0 mm, lies on the portion of the curve abovethe transition region. After the predilation the balloon is deflated andthe catheter 10 is removed from the vessel 14.

The next step is to deliver the stent to the lesion. In a firstembodiment of the process, a separate stent delivery catheter of anyconventional type is used to deliver the stent to the lesion, installthe stent in place across the lesion, and further dilate the lesion to alarger diameter, typically 2.75-4.0 mm. The delivery catheter is thenwithdrawn to leave the stent 17 in place across the dilated lesion, asshown in FIG. 2. Occasionally as indicated in FIG. 2 the stent is notfully seated or can move somewhat after installation if the installationprocess is discontinued at this point.

To assure that the stent is firmly seated in the lesion so that itcannot move and to additionally reduce occurances of restenosis andthrombus formation, in this embodiment of the inventive process, afterthe delivery catheter has been removed, catheter 10 is reinserted andexpanded to a retouch pressure, typically above 9 atm and preferably inthe range of 12-20 atm.

Alternatively, catheter 10 may be employed as a delivery catheter. Inthe specific embodiment illustrated in FIGS. 3-4, an unexpanded stent 18has been mounted on the catheter 10 over balloon 12 after catheter 10has been used to predilate the lesion and has been removed from thelesion. Catheter 10 is then reinserted into the vessel 14 and locatedacross the lesion (FIG. 3). Balloon 12 is then reinflated as shown inFIG. 4 to expand and install the stent 18 and to dilate the lesion. Thepressure employed is one which inflates the balloon to a diameter abovethe transition region and therefore the same balloon as used inpredilation can be used to deliver the catheter and dilate the lesion.Further, because the balloon 12 follows a low compliance curve above thetransition region, the pressure can safely be increased above 12 atm soas to firmly seat stent 18 without having to undergo “retouch.”Typically the balloon 12 will be capable of inflation to at least ashigh as 20 atm.

FIG. 5 depicts the stent 18 in place after high pressure dilation. Asimilar result is obtained if the catheter 10 is used for predilationand for “retouch” but not for stent installation. It should be notedthat the specific configuration of the stents 17 and 18 is not criticaland two different configurations have been depicted merely to indicatethat different configurations may be employed in either embodiment ofthe inventive installation process. The particular configurationsemployed may be reversed or another stent configuration, includingballoon expandable stents and self-expandable stents, may be substitutedwithout departing from the invention hereof.

Thus unlike the prior art methods for accomplishing the same sequencesof predilation, stent delivery/dilation and high pressure seating or“retouch,” a separate catheter is not required to be used in the finalhigh pressure seating step from the catheter used in the predilationstep. This significantly reduces the cost of the procedure, since thecatheter costs are a significant part of the overall cost of theprocedure.

The stepped compliance curve balloons should be made of a thermoplasticpolymer material which has a high strength, and gives a low complianceballoon at pressures above about 15 atmospheres. For purposes of thisapplication “low compliance” is considered to correspond to a diameterincrease of no more than 0.1 mm per increased atmosphere of pressure,preferably less than 0.06 mm/atm. Suitably the balloon polymer ispoly(ethylene terephthalate) (PET) of initial intrinsic viscosity of atleast 0.5, more preferably 0.7-0.9. Other high strength polyestermaterials, such as poly(ethylene napthalenedicarboxylate) (PEN), nylonssuch as nylon 11 or nylon 12, thermoplastic polyimides and high strengthengineering thermoplastic polyurethanes such as Isoplast 301 sold by DowChemical Co., are considered suitable alternative materials. Desirablythe balloon is blown in a way which will give a wall strength of atleast 18,000 psi, preferably greater than 20,000 psi. Techniques formanufacturing balloons with such wall strengths are well known.

After being blown, the balloon is provided with a stepped compliancecurve by annealing the balloon for a short time after blowing at apressure at or only slightly above ambient and at a temperature whichcauses the blown balloon to shrink. The process is described in U.S.Pat. No. 5,348,538. However, the balloons of the invention are desirablyconstructed with a greater difference between the low pressure and highpressure linear regions of the compliance curve so that the transitionbetween the two regions results in a step-up of diameter of the balloonof at least 0.4 mm. This is accomplished by blowing the balloon to thelarger diameter and then shrinking to a greater extent than was done inthe specific illustrative examples of U.S. Pat. No. 5,348,538. Theamount of shrinkage is controlled by the pressure maintained in theballoon during annealing and the temperature and time of the annealing.For a balloon made from 0.74 intrinsic viscosity PET, the blowingpressure is suitably in the range 200-400 psi, and temperature issuitably in the range of 90-100° C., and the annealing pressure is inthe range of 0-20, preferably 5-10 psi at 90-100° C. for 3-10 seconds.

In a further aspect of the invention, the balloons employed in theinventive process are configured so that a first portion of the body ofthe balloon has a stepped compliance curve and the remainder of theballoon has an unstopped compliance curve, the low pressure regions ofthe compliance curves of both the first portion and the remainderportion(s) being generally collinear. By this means the length of theballoon which will expand and seat the stent will be smaller than thelength which is used to accomplish predilation. Since many stents are inthe 7-10 mm length range whereas predilation balloons are desirably15-20 mm or even longer, this shorter configuration for the portionwhich will step-up to a larger diameter (“hyper-extend”) is desirable sothat the hyper-extension will not overlap tissue which is unreinforcedby the stent. Two balloons of this preferred configuration are shown,mounted on catheters, in FIGS. 6 and 8.

In FIG. 6, the balloon 30 is shown in its fully expanded high pressureconfiguration, mounted on a catheter 28. The balloon is mounted on thecatheter at balloon end regions 31, 33, located on opposite ends of theballoon body. As shown schematically in FIG. 7, this balloon is blown ina mold of the general shape of the balloon in FIG. 6 and then theannealing step is performed on the enlarged portion 32 by dipping theballoon in the direction indicated by arrows 36 to level A in a bath ofheated water or other suitable heated fluid while the balloon ispressurized at low pressure, for instance 0-10 psi, so that only portion32 is annealed. After annealing portion 32 will be shrunken so that, theconfiguration of the balloon will be substantially linear and willexpand generally linearly until pressurized above about 8-12 atm. Athigher pressures, the portion 34 of balloon 30 will continue to expandalong the same generally linear curve but portion 32 will rapidly expanduntil the balloon configuration is restored to shape shown in FIG. 6,after which the expansion profile of portion 32 will level out again toa non-compliant curve but at a substantial increase in absolute diameterrelative to the diameter of portion 34. Balloons of this configuration,have been used to produce compliance curves as shown in FIG. 11.

It should be understood that while FIG. 6 shows portion 32 of balloon 30mounted distally on catheter 28, balloon 30 may instead be mounted withportion 34 mounted distally without departing from the invention hereof.

If the balloon of FIG. 6 is used to deliver and install the stent, thecatheter 28 will have to be backed up a short distance to center portion32 under the stent after expansion of balloon 30 sufficiently to bringit into contact with the lesion but before the balloon portion 32 isfully expanded to fully dilate the lesion and set the stent. This can beaccomplished by providing marker bands (not shown) on the portion of thecatheter shaft under the balloon to indicate the proximal and distalboundries of portion 32.

In the alternate embodiment of FIG. 8, the balloon 40, mounted oncatheter 38, has a hyper-extensible portion 42 located centrally on theballoon body. The balloon is mounted on the catheter at balloon endregions 41, 43, located on opposite ends of the balloon body. Therefore,after installation of the stent, the high pressure stent setting stepcan be performed immediately without repositioning the catheter andwithout risking damage to tissue unreinforced by the stent. This balloonis blown in a mold having a configuration which is substantially theshape shown in FIG. 8. To anneal and shrink portion 42 to the diameterof portions 44, 46, heating during annealing may be confined to thecentral portion 42, suitably by heating with a hot air stream, usingbaffles to protect the end regions 44, 46 from the air stream.Alternatively, as shown schematically in FIG. 9, the balloon 40 isdipped in the direction of arrows 47 to level A in a heated bath tofully immerse portions 42 and 46, until portion 42 has reached thediameter of portion 44. At this point portion 46 will be shrunk to adiameter less than portion 44. Balloon 40 is then dipped into a heatedbath in the direction of arrows 49 to level B so that only portion 46 isimmersed and then portion 46 is reblown to the diameters of portion 44and shrunken portion 42. This reblowing step may be accomplished eitherwith the aid of a mold or by free-blowing.

Referring now to the graph shown in FIG. 10, in which pressure inatmospheres is plotted on the x-axis and balloon diameter in millimetersis plotted on the y-axis. The compliance curves of several balloons havebeen manufactured in accordance with U.S. Pat. No. 5,348,538 and usefulin the practice of this invention have been plotted on this graph andcompared to a conventional 3.5 mm angioplasty balloon Q of the same PETmaterial. The stepped compliance curve balloons, X, Y and Z, plotted onthis graph had nominal diameters prior to being shrunk of 3.0, 3.5 and4.0 millimeters, respectively.

FIG. 11 is a graph of the compliance curves of a balloon of the typeshown as balloon 30 in FIG. 6. Curve 11 a is the compliance curve ofportion 32 of balloon 30 and curve 11 b is the compliance curve of theportion 34 of balloon 30. The balloon was made from PET of 0.74intrinsic viscosity and, after blowing had a body wall thickness of0.0013 inches. Portion 32 thereof was annealed by dipping in a 95° C.water bath for 5 seconds, while pressurized at 10 atm pressure, toshrink portion 32 to the diameter of portion 34. The balloon was thenmounted on a catheter and the compliance curve obtained by incrementallyinflating the balloon until burst, measuring the diameter of bothportions 32 and 34 at each incremental pressure.

With regard to definitions, FIG. 11 can be referred to for illustrationof what is meant by “generally linear” with reference to the portions ofcurve 11 a between 3 and 10 atm and again between about 13 and 26 atm.Curve 11 b is considered generally linear through out its entire length.“Generally collinear” is considered to encompass divergences between twocurves of no more than about 0.2 atm, preferably less than 0.15 mmdivergence between the two curves. Curves 11 a and 11 b are “generallycollinear” in the range from 3 atm to about 10 atm.

The invention may also be practiced by use of dual layer balloons suchas described in co-pending U.S. application Ser. No. 08/243,473, filedMay 16, 1994 as a continuation of now abandoned U.S. application Ser.No. 07/927,062, filed Aug. 8, 1992, incorporated herein by reference,and in U.S. Pat. No. 5,358,487, incorporated herein by reference.Suitably both balloons of the dual layer balloons are low complianceballoons designed with the outer balloon portion larger by at least 0.25mm than the inner portion and the inner balloon designed to burst at apressure below about 15 atm so that the compliance curve follows theinner balloon portion until it reaches burst diameter and then, afterthe inner balloon bursts, the outer balloon becomes inflated and can beexpanded to a larger diameter than the burst diameter of the innerballoon.

Although the present invention has been described in terms of specificembodiments, it is anticipated that alterations and modificationsthereof will no doubt be come apparent to those skilled in the art. Itis therefore intended that the following claims be interpreted ascovering all such alterations and modifications as fall within the truespirit and scope of the invention.

I claim:
 1. An assembly comprising: a) a catheter having a distal end;b) an unexpanded inflatable balloon mounted near the distal end of thecatheter, the balloon having first and second end regions adhered to thecatheter and an inflatable balloon body there between; and c) a hollowexpandable stent having an initial stent diameter, an initial stentlength and a pair of ends, the stent being mounted on the catheter overthe balloon, the stent being expandable by the balloon to a second stentdiameter larger than the initial diameter, the stent initial lengthshortening during expansion so that at said second stent diameter thestent has a second stent length shorter than said first stent length,and the balloon, when inflated at an effective inflation pressureeffective to cause the stent to expand to said second diameter, hasfirst and second adjacent longitudinal stepped body portions withdifferent diameters, the second stepped body portion diameter beinggreater than the first body portion diameter and having a length andposition such that it does not extend beyond the ends of the stent atsaid second stent diameter, a first tapered diameter transition regionextending from said first end region to said first stepped body portion,a second tapered transition region extending between said first andsecond stepped body portion and a third tapered transition regionextending between the second stepped body portion and the second endregion.
 2. An assembly as in claim 1 wherein the second stepped bodyportion, when the balloon is inflated at said effective pressure, has alength and position such that it does not extend beyond the ends of thestent at said second stent diameter.
 3. An assembly as in claim 2wherein the stent initial length shortens during expansion so that atsaid second stent diameter the stent has a second stent length shorterthan said first stent length.
 4. An assembly as in claim 1 wherein theballoon is made of a thermoplastic polymer selected from the groupconsisting of polyesters, nylons, thermoplastic polyimides andthermoplastic polyurethanes.
 5. An assembly as in claim 4 wherein theballoon is made of a polyester selected from poly(ethyleneterephthalate) and poly(ethylene napthalenedicarboxylate).
 6. Anassembly as in claim 1 having a burst pressure of greater than 15 atmand a compliance curve according to which the balloon expands at a rateof no more than 0.1 mm/atm from 15 atm to burst.
 7. An assembly as inclaim 5 wherein the balloon wherein according to said compliance curvethe balloon expands at a rate no more than is no more than 0.06 mm/atmfrom 15 atm to burst.
 8. An assembly as in claim 1 wherein the balloonis mounted on the catheter such that the balloon second stepped bodyportion is distal to the first stepped body portion.
 9. An assembly asin claim 1 wherein the balloon is mounted on the catheter such that theballoon second stepped body portion is proximal to the first steppedbody portion.
 10. The method for introducing a stent into a vessel at alesion site, the method comprising the steps of: introducing into thevessel an assembly as in claim 1 and manipulating the assembly so thatthe stent crosses the lesion, and subsequently inflating the balloon toan effective pressure whereby the stent is expanded to a second stentdiameter.
 11. The method as set forth in claim 10 wherein the balloon ismade of a thermoplastic polymer selected from the group consisting ofpolyesters, nylons, thermoplastic polyimides and thermoplasticpolyurethanes.
 12. A method as set forth in claim 11 wherein the balloonis made of a polyester selected poly(ethylene terephthalate) andpoly(ethylene napthalenedicarboxylate).
 13. An assembly comprising: a) acatheter having a distal end; b) an unexpanded inflatable balloonmounted near the distal end of the catheter; and c) a hollow expandablestent having an initial stent diameter, an initial stent length and apair of ends, the stent being mounted on the catheter over the balloon,the stent being expandable by the balloon to a second stent diameterlarger than the initial diameter, wherein the balloon has been molded ina configuration providing adjacent first, second and third steppedlongitudinal body portions of the body with different diameters, thesecond stepped body portion diameter being greater than the first andthird stepped body portions, the stent located such that the balloonsecond longitudinal stepped body portion, when the balloon ispressurized at an effective inflation pressure, expands the stent tosaid expanded stent diameter, and the balloon first stepped portion,when the balloon is pressurized at said effective inflation pressure hasa diameter less than the diameter of the balloon second stepped portion.14. An assembly as in claim 4 wherein the balloon is made of athermoplastic polymer selected from the group consisting of polyesters,nylons, thermoplastic polyimides and thermoplastic polyurethanes.
 15. Amethod for introducing a stent into a vessel at a lesion site, themethod comprising the steps of: introducing into the vessel an assemblyas in claim 13 and manipulating the assembly so that the stent crossesthe lesion, and subsequently inflating the balloon to an effectivepressure whereby the stent is expanded to an expanded stent diameter.